For example, a GE-PON (Gigabit Ethernet-Passive Optical Network) defined by IEEE Std 802.3 (IEEE Standard for Information technology—Telecommunications and information exchange between systems-Local and metropolitan area networks—Specific requirements, Dec. 9, 2005) has been known as a technique related to an optical access network according to the present invention (for example, see Non-Patent Document 1).
The GE-PON disclosed in Non-Patent Document 1 transmits an IEEE802.3 frame (Ethernet frame) at a gigabit rate.
As shown in FIG. 1, the GE-PON includes a central unit (OLT′: Optical Line Terminal), a plurality of remote units (ONU′: Optical Network Unit), and an optical splitter (SP) provided therebetween.
In the GE-PON shown in FIG. 1, a downstream optical continuous signal transmitted from the central unit (OLT′) is distributed to the remote units (ONU′) by the optical splitter (SP). Then, each of the remote units (ONU′) receives the downstream optical continuous signal distributed by the optical splitter (SP) all the time.
The inventors have proposed an optical access network before the present invention (for example, see Patent Document 1).
The optical access network disclosed in Patent Document 1 is referred to as a GE-OSAN (Gigabit Ethernet-Optical Switched Access Network).
As shown in FIG. 2, the GE-OSAN disclosed in Patent Document 1 includes a central unit (OLT′), remote units (ONU′), and an optical switching module (OSM′) provided therebetween.
The optical switching module (OSM′) uses a frame format defined by the IEEE802.3 standard to perform optical packet switching in the unit of gigabit Ethernet frames.
Therefore, each of the remote units (ONU′) needs to receive an optical burst signal, which is an output signal of the optical switching module (OSM′), at a high speed.
For example, it is considered that each of the remote units (ONU′) receives the optical burst signal using the technique used in the central unit (OLT′) of the GE-PON shown in FIG. 1.
In the upstream communication of the GE-PON, a plurality of remote units (ONU′) transmit data to one central unit (OLT′). Therefore, it is necessary to avoid the collision of upstream signals.
Therefore, the central unit (OLT′) designates a transmission start time and a transmission duration time to each of the remote units (ONU′) to avoid the collision of the upstream signals.
Accordingly, the upstream signal is an optical burst signal, and the central unit (OLT′) includes a receiving circuit that receives the optical burst signal from each of the remote units (ONU′).
The light intensity of the optical burst signal received by the central unit (OLT′) depends on the transmission distance between the remote unit (ONU′), which is a source, and the central unit (OLT′). Therefore, when the transmission distances between the remote units (ONU′) and the central unit (OLT′) are different from each other, the light intensities of the optical burst signals received by the central unit (OLT′) from the remote units (ONU′), which are sources, are different from each other.
Therefore, as shown in FIG. 3, the receiving circuit of the central unit (OLT′) includes an ATC (Automatic Threshold Control). The ATC detects the maximum value of the light intensity of each signal, and sets half the detected maximum value as a threshold value.
In the IEEE802.3 standard, the ATC detects the maximum value of light intensity, and compensates the time required to determine the threshold value based on the detected maximum value. Therefore, a guard time of 400 nanoseconds is defined.
However, the optical burst signal received by the remote unit (ONU′) of the GE-OSAN shown in FIG. 2 is different from the optical burst signal received by the central unit (OLT′) of the GE-PON shown in FIG. 1 in physical conditions.
That is, there is an idle state, such as IFG (Inter-frame Gap), between the Ethernet frames in the frame format defined by the IEEE802.3 standard used in the GE-OSAN.
Therefore, the optical switching module (OSM′) operates an optical switch during the IFG to perform optical packet switching while maintaining the Ethernet frame format.
However, the shortest time of the IFG is defined to 96 nanoseconds. If optical switching is performed at a middle point of the IFG, the guard time is about 48 nanoseconds, which is short. As a result, the receiving circuit including the ATC shown in FIG. 3 is likely not to satisfy the burst reception conditions of the remote unit (ONU′) of the GE-OSAN.
Therefore, it is necessary to develop a receiving circuit capable of satisfying the burst reception conditions of the remote unit (ONU′) of the GE-OSAN shown in FIG. 2.
In addition, as a related art of the present invention, an optical subscriber network has been proposed in which a space-division optical switch that switches optical signals during the same time slot is provided in a remote terminal (RT) provided between a line terminal apparatus (CT) of an exchange center and each subscriber network terminal apparatus (ONU) (for example, see Patent Document 2).
Further, a signal transmission system has been proposed in which a transmitting apparatus encodes each signal sequence such that a DC balance is obtained, adds an error-correcting code to the encoded signal, and outputs the signal to a receiving apparatus, and the receiving apparatus corrects the error of the signal sequence received from the transmitting apparatus and decodes each signal sequence that has been encoded such that a DC balance is obtained (for example, see Patent Document 3).    Patent Document 1: JP-A No. 2006-140830    Patent Document 2: JP-A No. 10-70509    Patent Document 3: JP-A No. 2003-318865    Non-Patent Document 1: IEEE Std 802.3, “IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements”, Dec. 9, 2005)